Signal compression and expansion arrangements in electric communication systems



Feb. 26, 1952 B. B. JAcoBsEN SIGNAL COMPRESSION AND EXPANSION ARRANGEMENTS IN ELECTRIC COMMUNICATION SYSTEMS 6 Sheets-Sheet l Filed Jan. 14, 1949 INVENTOR BENT U 0W fqCOSE/V ATTORNEY Feb. 26, 1952 B. B. JACOBSEN 2,585,325

SICNAL COMPEESSION ANO EXPANSION ARRANGEMENTS IN ELECTRIC COMMUNICATION SYSTEMS Filed Jan. 14, 1949 6 Sheets-Sheet 2 FROM F/G 6' FROM FPOM F/G. 6.

FROM 29 FROM32 FIG-J.

INVENTOR BENT BULW f/JCOBSEN mmm ATTORNEY Feb. 26, 1952 B. B. JACOBSEN 2,586,825

SIGNAL CCMPRESSICN AND EXPANSION ARRANGEMENTS IN ELECTRIC COMMUNICATION SYSTEMS Filed Jan. 14, 1949- e Sheets-Sheet 3 INVENTOR BEN T E/OM/ JFCf/V ATTORNEY Feb. 26, 1952 B. B. JAcoBsEN 2,586,825

- SICNAL COMRRESSION ANO EXPANSION ARRANGEMENTS IN ELECTRIC COMMUNICATION SYSTEMS 'Filed Jan. 11;, 1949 6 Sheets-Sheet 4 Feb. 26, 1952 B JACOBSEN 2,586,825

B. SIGNALIIIOgIl-DRESSION AND EXPANSION ARRANGEMENTS ECTRIC COMMUNICATION SYSTEM Flled Jan. 14, 1949 s 6 Sheets-Sheet 5 INVENTOR BENT 8(10 W 77960865 N ATTO R N EY 'Filed Jan. 14, 1949 Feb. 26, 1952 B. B. JAcoBsr-:N 2,586,825

SIGNAL COMPRESSION AND EXPANSION ARRANGEMENTS IN ELECTRIC COMMUNICATION SYSTEMS 6 Sheets-Sheet 6 naar. plv/afp Img vu'ssim sn.

,2, INVENToR .5E/vr ez/ow JHcoasEA/ ATTORNEY Patented Feb. 26, 1.952'

SIGNAL .CDMBRESSION AND EXPANSION ARRANGEMENTS IN ELECTRIC COMMU- Bent Bulow Jacobsen, London, England, assigner to `International, "Standard Electric Corporation, Neur Yo'rlg, N. rjafcorporatioiifof pelaware" Application January 14, 1.949, Serial No. 74,224

' incrementan January 1c, 194s (c1. 17e-i5) Claims. l

The present invention relates to arrangements for compression and expansion of the signal arri plitude in electric communication systems.

The invention is particularly useful in pulse code modulation systems though it is not conned to such systems, and could be applied to systems in which the transmission of the communication signals does not involve pulses at all.

Electric pulse code modulation systems of communication are described in United States patent specification No. 2,272,070, In these systems the signal wave is scanned at sufliciently frequent instant of time, and the amplitude at each instant is measured according to a stepped amplitude scale. The nearest stop corresponding to the signal amplitude is then expressed by a group of pulses according to somer specified code, and these pulses are transmitted at specified instants to the receiver. The pulses are decoded at the receiver in order to recover the corresponding scale value of the signal amplitude, and the recovered amplitudes are reassembled to form a signal wave similar'to the original signal waves. The received wave is moreor--less-like the original wave, according as the ,amplitude scale has more or fewer steps.

It is clear that the numberof code elements necessary to represent the signal amplitude Will be greater, the greater the number of different amplitudes which must be represented by the code.

One coding system which has been proposed is provided by groups of n pulses eachof which may represent m conditions or amplitude'steps. This would enable a scale of amplitudes lcon'- taining mn steps to be employed. 'A

rlhis could for example be dealt with in practice by a code system of the abovegtype in which m=2 and 1L=5, the two conditions represented by m being, for example, the presence and absence of a pulse at the instant correspondingto that pulse. This would allow 32 amplitudes lto be expressed by the code.

It is doubtful, however, whether 32 amplitude steps would enable the signal wave to be reprdduced with sunicient fidelity at the receiving end for iirst class commercial communication systems, and certainly not for high quality systems such as broadcasting systems. Elf? more than 32 steps are required, at least onemorecode element would normally be needed. -Henceif n=6` and m=2, 64 steps would be available. Clearly, the additional code elements will increase the frequency bandwidth necessary for transmission of the code'pulses. It is therefore one objectof the present invention to provide an arrangement by which the number of amplitude r'steps maybe eiectively increased either without any increase in the number of code elements employed, or at least with a much smaller increaseV than would be necessary when the numberof steps is determined by the formula mn.

I,It has already been mentioned that the invention is not restricted to a system on which the communication signals are conveyed by pulse code 'modulation methods, but can be applied to any system of electric communication.

4According to the broadest feature of the invention, therefore, the signal wave is subjected to'a` process'by which the range of amplitude variation Vismodiiied before transmission and is subectdatf the receiving end to a correcting process which restores the range of amplitude variation 'to 'its loriginal value; and the control VofA the correcting process is e'lected by means of `information conveyed in discrete steps, according toa given scale from the transmitting end tothe receiving end by modulation of a series of electric pulses. 1 'The'manne'r in which the principal obj-ect of the invention is achieved depends on the fact Vthatthe general level of a signal such as is ordinarily conveyed'over a communication circuit varies 4at a rate which is slow compared with the instantaneous amplitude variations. For a signal ,at Vconstant level, in a pulse code modulation system, the amplitude variations from one scanning instant to another are confined to a range between thepositive and negative peak amplitudes and can be reproduced sufhciently accuratelylonuan amplitued scale with comparatively few stepsi Forexample, a total of 16 steps (8 positive" and 8 negative), might be sufhcient.

The peaksignal level, however, varies more slowly over a l,wide range. In one arrangement for carrying out the invention therefore, the signal wave is rst passed through a variable attenuator (or equivalent device) which is automatically controlled by a signal level measuring device of some conventional type in such manner that the output signal peak amplitude does notV exceed the amplitude allowed for the constant level system.'

The attenuation of the attenuator will vary over a suitable range in a sufficient number of steps, for example, there could be 32 steps each of 1.5 decibel, giving a total range of 48 decibels.

The setting of the attenuator is indicated by an auxiliary group of code or digit pulses, in which for instance 11:5 and 171:2. Since, however, as already pointed out, the signal level varies slowly, it is suicient to reset the attenuatoi` and communicate the setting to the receiver at considerably longer intervals than are required for the case of the transmission of the instantaneous signal amplitudes. For example, assuming that four code pulses are used as a principal code group to transmit the instantaneous signal amplitudes, and that it is sufficient to re-adjust the attenuator setting, for instance, ve times less frequently than to transmit the communication signal amplitude, then each grou-p of four code pulses could be prefixed or followed by one of the auxiliary code pulses. The full information regardingI the attenuator setting will therefore be given by each ve successive code groups. The settings will be made at the time of reception of the last auxiliary code pulse.

It will be seen that in the example given, eiectively 512 amplitude steps (that is 16 times 32) can be denoted by the same number of code pulses which would be required to denote only 32 steps according to the simple code system already in use.

It will be understood that at the receiver, suitable means will be provided to decode the auxiliary code pulses and the principal code pulses. The signal wave after being reconstituted by any suitable method from the groups of four code pulses is passed through another attenuator controlled by the information obtained from the auxiliary code pulses, so that the received signal is restored to its original level.

It will be understood that the term attenuator means any device having an adjustable transmission equivalent, and may be, or include, a variable gain amplifier.

It will be evident that the auxiliary code pulse may be combined with the principal code pulses in various other ways. For example, they could the effect on the signal amplitude at the sending end can be arranged to be exactly and synchronously compensated at the receiving end.

It should be further pointed out that the auxiliary control of the signal level according to the invention has a considerable advantage in reducing the loud noises which are liable to result from the code errors produced by the accidental loss or addition of code pulses due to noise or interto arise from the scale steps with the ordinary code modulation system is avoided, since the main channel signal level is always automatically adjusted to a nearly constant value before coding. These advantages may be as important as the equivalent saving in bandwidth which the system of the invention can produce.

In multichannel pulse code modulation systems the auxiliary code groups corresponding to the respective channels may be transmitted in a number of different ways. One method which is used in a preferred embodiment to be described in detail'later, consists in transmitting three principal code groups corresponding to channels l to 3, then an auxiliary code group corresponding to one of the main channels, then three more principal channel code groups, then an auxiliary code group corresponding to another main channel and so on. It is evident that by interspersing the auxiliary code groups among the principal channel code groups according to this type of plan, various other frequency relations of the auxiliary code groups with respect to the principal channel code group frequency can be obtained.

be transmitted as a close group of pulses before or after a suitable succession of groups of the principal code pulses.

The example given above is only one instance of the manner in which the signal amplitudes could be coded according to the principles of the invention. The principal and auxiliary code groups might have any number of pulses, and the amplitude and signal level scales might have any desired number of steps which might be the same or different. the signal amplitude and signal level are scanned could have any convenient value, and other systems of coding could be used. l

While it has been assumed above that the prin The ratio of the intervals at which i It will be understood that although the invention ywill be described with reference to pulse code modulation systems where its most imporl i tant application may lie, it can also be applied to any other type of pulse modulation systems, for example to time phase or duration modulation pulse systems. In this case auxiliary groups of code pulses representing the signal level ad- A justments at the transmitting end could be transmitted to the receiver suitably interspersed among the time modulated pulses, enabling the complementary adjustments to be made at the receiver.

The invention is also not limited to communication systems employing pulses. In the wellknown compandor systems employed with voice frequency transmission systems, the signal amplitude is compressed before transmission, that is, the amplitude variations are made to occur within narrower limits, by passing the signal wave through a device including some kind of contransmit the auxiliary code groups for all the channels over a circuit separate from the circuit or circuits used for the principal code groups of the channels.

It may be pointed out that although the auxiliary signal level control is in steps, this need not produce any signal distortion additional to that A` due to the steps of the principal control. beeause trolled attenuation network which produces the effect of increasing attenuation as the general signal amplitude increases. At the receiving end an expanding network of similar character restores to normal the general amplitude variations. The two networks are designed to be complementary to one another.

When the principles of the present invention are applied to this type of system, the signal wave will be passed through an attenuating devicethe attenuation of which is lvaried in some desired manner in accordance with the signal amplitude variations, and the signal level then periodically coded and is conveyed by code pulses to'control the transmitting and receiving attenuators.

The invention will be described with reference to a particular embodiment Aillustrated in the accompanying drawings in which:

Fig. 1 shows a schematic circuit diagram of the transmitting end of a communication system according to the invention;

Fig. 2 shows a block schematic circuitof the distributors diagrammatically shown in Fig. i;

Fig. *3 shows graphical diagrams used to explain the timing arrangements of 1;

Fig. 4 shows a schematic circuit diagram ci the receiving end of the system;

Fig. 5 shows a modification of Fig. l;

Figs. 6 and 7 respectively show block schematic circuit diagrams of the synchronising arrangements at the transmitting and receiving ends of the system; and

Fig. 8 shows details of the anticipator circuit shown in Fig. 1.

In order to illustrate several aspects of the invention, a particular system employing pulse code modulation will be assumed.

In this system there are altogether sixteen channels, of which twelve are used for conveying the communication signals, three are used as auxiliary channels, and one is used as a synchronising channel.

Each channel is represented by a tra-in of pulse code groups, the groups being repeated at a frequency of 8,000 groups per second, which will be called the channel repetition frequency. The trains corresponding' to the sixteen channelsl are interleaved in the usual way before transmission, so that over the communication circuit l28,000 pulse groups are transmitted per second. This will be called the groups repetition frequency.

The interleaving of the channel pulse trains is arranged so that in each signalling period the pulse code groups follow in the order S, l, 2, 3, C, 4, 5, 6, D, 7,A 8, 9, E, 10, 11, 12; in which groups corresponding to the various communication channels are numbered 1 to 12, S represents a synchronising group, and C, D, E represent groups corresponding to the three auxiliary chan. nels.

In one arrangement for carrying out the invention, the signal wavefor each channel before being coded is passed through anV amplifierwhose gain is reduced as the general signal amplitude -level increases (andvice versa) in order to.

produce amplitude compression., or reduction in the range of variation of the amplitude 0f the signal wave.

Then the information regarding the compres-` sion at the transmitting end of lthe system for channels 7, 8, 9 and 10 is conveyed to the `receiver over the auxiliary channel C in the following Way. The first code group transmittedcver channel C provides information as tothe Again of the amplifier of channel 7, the second suc-h group, of channel 8, and the thirdv and fourth groups, of channels 9 and 10 respectively. Then the fifth group refers to channel 7 again and so on. Thus while the normal code groups for any given `channel are transmitted 8,000vtimes per second, the code groups which indicate the degree `of vconipression for that channelaretransmittedv only 2,000 times per second.

The auxiliary channel;-D.-dea1s -inza-slmilarivgy;

withthe communication channels 11, 12, 1 and 2 and the auxiliary channel E deals with the communication channels 3, 4, 5 and 6. Other allocations of channels could obviously have been used.

The code groups corresponding to the synchronising channel S convey a periodic signal employed to synchronise the apparatus at the receiver as will be explained in detail later.

Itgwill be assumed that the code employed for allchannels will be a four-digit binary code, each element of which is represented by the presence or absence of a. single pulse, and the code groups will be transmitted in succession without any gaps between. Thus there will be 512,000 instants per second at which a digit pulse may be present or absent. The digit pullse repetition frequency ofthe system is therefore 512,000.

It will be understood that a four-digit code will provide an amplitude scale with 16 steps for coding the signal wave after compression. It will also be suiicient to deal with 16 gain steps on the compression amplifier, so that the effect is equivalent to providing an amplitude scale with 256 steps, which without the compression arrangement according to the invention would need an 8-digit code.

The transmitting arrangements of the embodiment just described in general terms are shown in Fig. 1. The main channel collector or distributor is shown diagrammatically as a 16-pcint commutator 20, and it will be seen that the communication channels 1 to l2 are admitted sequentially in groups of three at a time, the groups being followed respectively by auxiliary signal and synchronising channels, designated C, D, E and S. The information for each of the auxiliary channels is fed from a corresponding one of the auxiliary signal channel collectors, 2l, 22, 23 represented diagrammaticallyas li-point commutator switches. For clearness Fig. 1 shows in detail only the arrangement for channel 10, and it will be seen that the signal wave is fed from the main channel hybrid coil 24 through the low pass lter 25 to a. further hybrid coil 26. From here the signal wave is divided between two paths, one of which leads to the point l0 of the commutator 20 through a delay circuit 27, a high frequency de-emphasis circuit 28 and a compressing amplifier 29 whose gain is controlled effectively by the signal level, and which is connected to the point I0 over conductor III. The other path includes elements for producing a control signal which controls the compression amplifier 29 and also the corresponding expansion amplier at the receiving end. These elements comprise full wave rectifier 30 followed by a low pass lter 3l which generate a, relatively slowly varying voltage corresponding to the varying energy content of the signal wave. The low pass lter 3l may, if desired, be followed by an anticipator circuit 32 which will be described later, but which could be omitted. The anticipator is connected to .point I0 of the commutator 2| over conductor H0.

The de-emphasis circuit 28 is a known type of circuit which attenuates the high frequency end of the signal band with respect to the low frequency end, and is used to reduce the high frequency energy content so that the main channel shall not be overloaded by the high frequencies. This unit lis .also not essential and could be omitted.

The delay network s2'lfmay beprovided for compensating the delay introduced by the low pass filter 3|.

The control signal voltage fed to terminal I of the auxiliary commutator 2| is collected in its turn and applied to point C of the channel commutator 20 for transmitting to the receiver the required auxiliary signal information for channel 10. The compressor unit 33 is preferably included between the commutators 2l and 20. The output from the channel commutator 20 is fed to a pulse coding unit 34 of any suitable type to which the appropriate digit pulses are also supplied over conductor 88 so that a train of code groups of pulses is obtained and can be fed to a transmission line or other communication medium. The element shown dotted at 84 is only used in a modification of this embodiment which will be described later, and should for the present be considered as omitted. A digit pulse marking circuit 109 is connected between the coder 34 and the line and is concerned only with the synchronising channel, and will be described later. It has no effect on any of the other channels.

The coding unit 34 might, for example, be of the kind described in Patent No. 2,272,070..

In order that the information conveyed by an auxiliary signal channel may be used to perform exactly complementary functions at the transmitting and receiving ends, the code groups actually transmitted are decoded at the transmitter by the decoding unit 35 in the same way that they will be decoded at the receiving end, and the recovered control signal is then used to control the compressing ampliiier of the corresponding channel at the transmitting end, which in n the case illustrated is channel l0. The decoding unit 35 could, for example, be similar to that described in the last-mentioned co-pending speciiication. The selection of the appropriate auxiliary channel information is performed by two further collectors or distributors shown diagrammatically as commutators 3B and 31. The dotted element 85 is not used in this embodiment and should be disregarded.

Commutator 31 corresponds to the auxiliary channel C and is similar to commutator 2|, and it will be understood that there will be two other commutators (not shown) similar to 31, for the auxiliary channels D and E. The commutator 36 has three outlet points C, D and E for selecting the appropriate one of the three commutators corresponding to 31, at the times when the commutator 20 is selecting the corresponding auxiliary control signals.

The point I0 of the commutator 31 is connected over the conductor I I2 to the gain control circuit oi the compression amplifier 29, and the gain of this amplier should preferably be adjusted in equal steps of for instance 3 decibels by the decoded control signal.

In order to obtain this result, the compressor 33, which should preferably have a logarithmic characteristic, is provided. In this case, the output voltage of the compressor would normally be of one sign only, and in order that it may employ the whole range of the coder 34, which will of course be designed to deal with positive and negative voltages, the output of the compressor should be biased so that it covers a range of equal positive and negative values.

Inserted between the commutator 31 and the compressing amplifier 29 is a holding condenser 38 which maintains the control voltage derived from the code sent out on the auxiliary channel C for. four. channel periods..

In order to avoid a large time constant associated with this condenser, an amplifier 39 with a low output impedance (such as a cathode follower ampliler) may be inserted after the decoder 35.

The commutator 36 will rotate at the same rate as the commutator 20 (8,000 revolutions per second) but will have only three outlets corresponding in timing to the inlets C, D, E of commutator 20.

The commutators 2l, 22, 23, 31 (and the other two, not shown, similar to 31) will all rotate four times more slowly, namely, 2,000 revolutions per second, and each will have only four steps corresponding to the four channels which it serves.

A simplified arrangement of the transmitter is also indicated in Fig. l. The compression amplifier may be controlled Without the complication of coding and decoding, by making the connection indicated by the dotted conductor 82. In that case the coder 35 is not required and can be omitted. It is to be noted however, that since there are now no quantising steps at the transmitter and since the adjustment of the corresponding expansion amplier in the receiver must be quantised owing to the coding, there may exist an error of compensation of the compression of nearly one quantising step. Thus the quantising steps on the auxiliary channels should preferably be somewhat smaller than 3 decibels when the direct connection 82 is used.

The digit pulse marking circuit lil-9 in Fig. l is provided for the purpose of identifying the first digit element of each synchronising period so that the distributors at the receiver (which will be described later) may be not only synchronised with the distributors at the transmitter, but may also be locked properly in step. To this end, the first digit element of each of the code groups of the synchronising channel is identified by modulating it so that a digit pulse is always present in the first element of alternate code groups and always absent in the case of the others.

This element can be recognised at the receiver by means to be described later, and denitely marks the phase of the channel recurrence periods.

The synchronising channel is also modulated with a sine wave or other periodic signal having a frequency equal to that of the auxiliary distributors (2,000 cycles per second).

The marking circuit |09 consists simply of an adding circuit of conventional type in which digit pulses from the coder 34 are combined with alternately positive and negative marker digit pulses generated in the synchronising circuit, Fig. 6, and supplied over conductor 94, Fig. l. The negative marker pulse cancels any digit pulse which may be present in the first element of the code group and the positive marker pulse places one there if it does not already exist. The adding circuit is followed by a conventional amplitude limiter or Slicer circuit which eliminates any negative marking pulses left behind and reduces all positive pulses to the same amplitude.

It will be understood that the marker pulses are timed to occur at a characteristic instant of each channel recurrence period. l

It has already been mentioned that the commutators shown in Fig. 1 are merely diagrammatic representations of suitable collector or distributor systems, and several Well known arrangements will suggest themselves to those skilled in the art. However, details are shown in Fig. 2 ofthepreferred form Yof 4the commutatingr arrangements and the timing sequence is illustrated in the graphs of Fig. 3. The commutator .'20 is shown to comprise sixteen similar vgate circuits of conventional type, one of which is designated ||3. These gate circuits are also numbered or lettered to correspond with Fig. 1. Thefoutputs of al1 these gate circuits are connected in common to a conductor which will be connected to the coder 34 of Fig. 1. The input terminal of `each of the gate circuits for the communication channels 1 to 12 is connected to the output ofthe compressing amplier 29 of the corresponding channel (Fig. 1 of course only shows one of these amplifiers, namely for channel 10). The input terminals of the gate circuits corresponding to the auxiliary channels C, D and E are connected to the corresponding commutator circuits on the left hand side of Fig. 2 through the compressors 33. The input terminal of the gate circuit corresponding to the synchronising channel -S will be connected to a conductor 98 which comes from Fig. 6. A phase adjusting circuit (not shown) may be included in this conductor.

The sixteen gating circuits are normally blocked, and are periodically opened at the proper times by gating pulses obtained from successive tappings of a delay network I I4 supplied by pulses at the channel repetition frequency from Fig. 6 over conductor 9| The tappings should be chosen so that the time interval between the opening of any two adjacent gating circuits 1s'- equal to the pulse group repetition period. `The group of curves :c in Fig. 3 shows the timing of the gating pulses for each of the sixteen channels. It will be seen that only one channel is open at a time.

The commutators 2|, 22 and 23 are represented by the series of 12 gating circuits ||5 on the left hand side of Fig. 2. These are also designated by the corresponding channel numbers 1 to 12. The commutator 2| for auxiliary lchannel C comprises the four gating circuits numbered 7 to l0; commutator 22 for auxiliary channel D comprises gating circuits Nos. 1, 2, 11 and 12, and commutator '23 for auxiliary channel E comprises gating circuits Nos. 3, 4, and 6.

In the case of commutator 22, the common output conductor of the gating circuits 7, 8, 9 and 10 is connected to the compressor 33 and thence to the gating circuit C of the commutator 20. The four input terminals of these four gating circuits are respectively connected to the anticipator circuits 32 of the corresponding channels.

(Fig. 1 shows only the anticipator circuit for channel 10.) The gating circuits for theother commutators 22 and 23 are treated similarly, the common output conductors being respectively connected through the corresponding compressors to the gating circuits of the auxiliary channels D and E, as shown.

The gating circuits H5 are controlled by Yfour trains of gating pulses obtained for the delay network 96 in Fig. 6 and supplied to the four conductors designated I, II, III and IV. These pulses have a repetition frequency of 2,000 per second which is equal to one quarter of the channel frequency. Their duration should be rather less than one channel period and each should synchronise with one of the channel periods. Each pulse controls three gating circuits, one from each commutator. Thus pulse No. I opens the gating circuits '2, 6 and I0, No. II opens circuits 5 and 9, No. III opens circuits 4, 8 and |2, and No. IV opens circuits 3, and

The group of four curves Y in Fig. 3 shows the last-mentioned gating pulses. Two of the pulses I. are shown to indicate that they appear during every fourth channel period, and that thepulses I, II,-III and IV follow one another in successive channel periods. As already explained, the first pulse No. I renders the auxiliary channels C, D and E respectively active for the auxiliary signals lof communication channels No. 10,

2 and 6; pulse No. II for channels 9, l and 5, and f so on.

The commutator 36 of Fig. 1 is represented in Fig. 2 by three gating circuits H6 corresponding to the'three auxiliary channels and designated C, D and E. 'I'Ihe input terminals are connected in common to the amplifier 39 of Fig. 1, and the output terminals are connected respectively to the commutator 31 and to the other two coresponding ones not shown in Fig. l. Since each of these three gating circuits he s to be open in time to receive the decoded auxiliary signal, it will be clear that it should be opened a short period after the corresponding auxiliary channel gating circuit is open, depending on the coding and decoding delays, which will probably be slightly more than one code group period. The gating pulses for the three gating circuits 6 will accordingly be taken from tappings on the delay network H4 as indicated, slightly more than one code group period after vthe tappings from which the gating pulses for the corresponding auxiliary channels are talren.` These gating pulses are shown in the group of three curves 2 in Fig. 3.

The commutator 3"! of Fig. 1 and the two others (not shown) which accompany it may comprise twelve gating circuits arranged exactly similarly to the gating circuits l5, Ibut it is not considered necessary to show them in Fig. 2. They can be controlled by the same gating pulses from conductors I, II, III and IV, provided that the coding delays ,are not too long.

From Fig. 3 it can be seen that each of the gating pulses of group Y should be at least long enough to include the three auxiliary channel gating pulses C, D and E, and also the three gating pulses of group Z which occur during the corresponding channel period, but they should not be so long as to overlap one another.

Fig. 4 shows diagrammatically the arrangement at the receiving end of the system. The incoming train of pulse code groups is applied through a re-shaping circuit 40 to a decoder unit 4i which may be similar'to the decoding unit 35 of Fig. 1. The re-shaping circuit 40 is a circuit commonly used at the receiving end of a pulse system for cleaning up the pulses which have become distorted by transmission overvthe system, and may consist of an amplifier with amplitude limiting arrangements of conventional type for cutting out sliceszof the amplied pulses at a suitable level in order to produce pulses of improved shape. Regeneration of the pulses is assumed to be affected in the decoder 4| if this should be necessary. The output from this decoding unit is then fed to a commutator 42 similar to the commutator 20 associated with the transmitter circuit, Fig. l By this means the pulses produced at the output of the decoding circuit are sorted into separate channels to provide the original modulating signals. The auxiliary signal pulse trains provided atpoints C, D and E are further subdivided into trains corresponding to specic channels by means of auxiliary channel distributors 43, 44 and 45 respectively, which correspond to the distributors 2 l, 22, 23 of Fig. l. In Vthe case of channel l0, the auxiliary channel C is concerned, and therefore the point lil of the distributor 43 is connected through a low impedance amplifier 83 to a holding condenser 41 and thence to the expansion amplifier 48 of channel I0 which is connected to the point lil of the distributor 42. The gain of the expansion amplifier will be controlled by the signal received over channel C in such manner that the sum of the gains of the amplifiers 48 and 29 (Fig. l) is constant, so that the general signal level is restored to the proper value.

The amplifier unit 48 is connected to a high frequency emphasis circuit 49, which should be exactly complementary to the de-emphasis circuit 28 of Fig. l, and thence through channel amplier 53 and a low pass filter 5I to the channel hybrid coil 52.

The commutators 42, 43, 44 and 45 in Fig. 4 perform similar functions to the commutators 20, 2 l, 22 and 23 of Fig. l, and accordingly they can be provided in the manner explained with reference to Fig. 2, with the gating circuits H6 omitted, since they are not required at the receiver. The compressors 33 will also be omitted. The gating circuits -in Fig. 2 will, of course, have to be designed to transmit in the opposite direction to that shown in Fig. 2.

In the case of the embodiment which has been described in the preceding paragraphs, the range of the signal wave amplitude variations is reduced before coding on a multiplicative principle, that is, the effect of the compression amplifier' is to multiply or divide all the signal amplitudes by a variable factor such' that the multiplied or divided amplitudes vary over a substantially constant range. According to ano-ther aspect of the invention, the amplitude compression can be carried out on a subtractive principle, that is, a voltage representing a proportion of the signal arnplitude, and re-determined at relatively infrequent intervals, is continuously subtracted from the signal voltage, so that the resulting range of amplitude variations is reduced. The term amplitude compression will be used to cover this process.

The arrangements at the transmitter for the subtractive system differ only slightly from Fig. l', and it will only be necessary to point out the differences. First, the rectifier 38 is not required, and will therefore be omitted, and' secondly, the compression amplifier 29 is replaced by a subtraction circuit of any suitable type in which the potential acquired by the condenser 38 isv effectively subtracted from the signal Wave voltage at the output of the de-emphasis circuit 28.

Furthermore, it is preferable to insert an amplitude compressor similar to 33 before the coder S4, and this is shown dotted at 84. A corresponding amplitude expander should be inserted after the decoder 35 as shown dotted at 85.

It will be seen that the instantaneous voltage of the signal Wave (after the upper end of the frequency band has been removed by the filter Si) is used to charge the condenser 38 when the distributor 3l makes contact with point l0 and distributor 33 makes contact momentarily with point C. The condenser 38 holds the charge for four channel periods until the distributors 36 and 3l again establish connection with the amplier 33, and is then charged to a new potential corresponding to the new auxiliary code signal, and holds it again for four channel periods, and so on.

The proportion of the signal voltage which is subtracted can be adjusted by suitably adiusting iii.

l2 the gain of the amplifier 39. Preferably practically all that part of the signal voltage which is due to the low frequency components, together with the voltage due to the effect of the anticipator circuit 32 should be subtracted.

It will be understood that the value of the voltage subtracted is conveyed to the receiver over the auxiliary channel C by the pulse code groups exactly as before. The receiver differs l from Fig. 2 only in that the expanding amplifier 48 is replaced by an adding circuit of any convenient type by which the voltage held in condenser 41 is added in correct proportion to the decoded signal voltage obtained from the com mutator 42, thus restoring the signal amplitude to the original value. If the amplitude compressor 84 is used at the transmitter, then a corresponding amplitude expander shown dotted at |00 should be inserted after the decoder 4|.

The transmitter and receiver of Figs. l and 4 may be also operated in a somewhat different manner. According to the method already described, the signal level of each communication channel is determined at the end of every four channel periods, and the value is signalled over the corresponding auxiliary channel to the receiver for adjusting the expansion amplifier accordingly. It is however, possible to signal only the changes in the signal level instead of the actual value, and in this case, when the signal level in any communication channel remains constant, no signals are sent over the corresponding auxiliary channel. incremental method and is applicable when' either the multiplicative or the subtractive sysd tem is used. Y

Fig. 5 shows the modification of part of Fig. lrequired for the incremental method. The ele' ments not shown in Fig. 5 are supposed to be the same as in Fig. 1. The chief difference is that the auxiliary voltage representing the signal level is obtained from the output of the compression amplier 29 instead of directly from the signal input circuit, and that the holding condenser 38 is replaced by an integrating circuit H8.

It will be understood that the compression amplier 29 operates to maintain a constant out-l put level within certain limits, whatever the input level may be, so that it is the changes in the signal level which will be determined at the out-l put of the compression amplifier. In o rder thatY the gain of the amplifier may follow the varia tions in the signal level, it is evident that these` changes must be integrated so that, for example',- a progessively increasing signal level will caiis'ey af progressively decreasing gain, While the output level remains nearly constant. This integration is carried out by the integrator H8.

However if the decoder 35 is of the type which provides a unipolar output voltage, it will .be necessary to bias the output so that it covers a range of equal positive and negative values in order that the integrator may operate as desired. The arrangement should be such that when the signal level has remained constant for a period', so that theV level at the output of the amplifier- 29` has adjusted itself within the proper limits,

the decoder will produce pulses of substantiallyy zero amplitude, and when the signal level then increases or decreases the decoder will produce positive or negative pulses, respectively. changes in signal level will then be integrated correctly by the integrator l I8.

In" the multiplicative system, when operated' by the incremental method, the bias of the corri-l This may be called the'l The 13 pressor 33, Fig. 1, should be such that when signals at a constant level have been applied to the channel for some time, the voltage applied to the auxiliary channel C is practically zero.

In the receiver circuit, Fig. 4, for the incremental method, the holding condenser 41 should be replaced by an integrating circuit similar to I I 8, Fig. 5. However, by the incremental method, there is a tendency for the adjustment of the expansion amplifier at the receiver to get out of step with the adjustments of the compression amplifier at the transmitter during a period when no signals are passing. This is due to the fact that in this condition the gains of the amplifiers tend to be driven beyond the limit of their range by the action of the integrators, and when signals return, the gain of the expansion amplifier may not be brought back to the proper value. To prevent this, a diode clamp circuit is attached to the output of the integrator I I8 in Fig. 5, consisting of a diode 120 biassed with a suitable source I2I to limit the output voltage of the integrator to a definite value. A similar diode and source (not shown) will be connected across the integrator which replaces the condenser 41 e' in the receiver, Fig. 4, but will be inverted, since the gains of the compression and expansion amplifiers operate in opposite directions. The potentials of the bias sources at the transmitter and receiver should be chosen so that the limit is simultaneously reached at both ends. Then the expansion at the receiver will pick up at the proper point on the return of the signals. It will be noted that this arrangementensures a proper alignment after an interruption to the circuit.

In the case when the subtractive system is used according to the incremental method, it is necessary that the output voltage of the integrator should be zero after a period of no signals, and this can best be ensured by providing the storage condenser of the integrator with a suitable leak resistance by which it can be discharged with a time constant which is not too short. A similar leak is applied at the receiver.

The synchronising arrangements at the transmitter and receiver are shown in Figs. 6 and '1 respectively. Everything is controlled by a master pulse generator 81 (Fig. 6) at the transmitter, which generates the digit pulses with a repetition frequency of 512,000 pulses per second. The digit pulses are supplied directly to the coding unit 34 (Fig. 1) over conductor 39. Pulses similar to these digit pulses, but with the channel repetition frequency of 8,000 pulses per second, are obtained by means of pulse divider 99, which divides by 64, and are used to perform two functions, namely, to provide the channel gating pulses and to generate a synchronising signal for transmission to the receiving end of the system. For the first purpose the pulses are lengthened to a duration corresponding to four digit pulses by means of the pulse shaper 90 and are supplied over conductor 9| to the delay network I I4 shown in Fig. 2. Y

For the second purpose, the pulse train at the output of the divider 89 is passed through another divider 93 which divides by two, and then through a pulse inverting circuit |22. Then the pulse trains obtained from units 89 and I22 are fed to a marker pulse generator 92 which produces a pulse train in which alternate pulses have positive and negative polarity. The generator 92 is merely an adding circuit by any suitable type, in which the inverted pulses occurring 4,000 times per second are added to the original pulses occurring 8,000 times per second. The amplitude of the invertedv pulses should be sumcient to override each alternate original pulse, thus converting it into a pulse of the opposite sign.

The alternately positive and negative pulses at the output of the generator 92 are supplied over conductor 94 to the unit |09 of Fig. 1, and operate to ensure that the rst digit pulse of each synchronising group is alternately present and absent, as already explained. A delay network I23 may be included in the conductor 94 in order to time the marked pulses with respect to the pulses at the output of the coder 34 (Fig. 1).

The pulse train at the output of the pulse Shaper is also .passed through another divider 94 which divides by four thus producing a pulse train having the gating frequency 2,000 per second required for the auxiliary channel commutators 2I, 22, 23, 31 etc. described in Fig. 1. The .pulses of this train are then given the required length indicated by curves Y of Fig. 3 by the pulse shaper 95, and are then fed to the delay network 96 from which are derived the four cor rectly phased trains of auxiliary pulses I, II, III and IV required for Fig. 2. The output from unit 94 is also fed to a band-pass lter 91 which extracts the fundamental frequency of this pulse train which is supplied to terminal S of the commutator 20 of Fig. 1 over conductor 98 for modulatingthe synchronising channel to provide means at the receiver for generating corresponding properly phased auxiliaiy channel gating pulses. The need for this can be seen from the fact that each auxiliary channel deals with four different communication channels in turn, and it is therefore necessary to ensure that the auxiliary channel gating arrangements at the transmitter and receiver are in step, so that each auxiliary channel is dealing with the same communication channel at both ends. As already mentioned, a phasing network |24 may be ineluded in the conductor r98.

Fig. 7 shows diagrammatically the synchronising arrangements at the receiver. The in coming train of code groups is supplied over conductor 99 (Fig. 4) (connected just in front of the decoder 4I) to a full wave rectifier 53 (Fig. 7) which ensures that the repetition frequency of the digit pulse train will be made available even if the incoming pulse train consists for instance only of alternate digit pulses or other unfavourable combinations, and is then fed to a bandpass lter 54 which is arranged to pass only the digit pulse frequency of 512,000 per second. This filter may be a crystal filter, or may be of the double heterodyne type to obtain more readily a very high degree of selectivity. The output from unit 54 is then fed to an amplifier 55 which is provided with an automatic volume control capable of handling the wide variation of signal amplitude liable to be Obtained from unit 55, when a large number of individual digit .pulses are absent from the incoming pulse train. The output from unit 55 is applied to a pulse generator 56, for generating a digit pulse train having a repetition frequency of 512,000 pulses per second. This pulse train is applied to the divider 58 which divides by 64 to provide a pulse train having the channel repetition frequency of 8,000 pulses per second.

For the purpose of bringing the receiver into step with the transmitter, a combining circuit 51 is introduced between the units 56 and 58. The

circuit 5l consists of any suitable circuit' for adding the pulses from the generator 5S to pulses obtained from the gate circuit 63 which will be ,mentioned later.

The pulses at the output of the divider 53 are applied to an output conductor l02 through a pulse Shaper lill which gives them the required duration rather shorter than the time occupied by one code group. It is assumed that the coder will have a storage condenser for holding the value of the signal voltage which has to be coded. Il not, the duration of the gating pulses should be slightly greater than the time occupied by the code group. The sixteen trains of channel gating pulses for the receiver commutator i12 (Fig. 4) are then obtained from a delay network (not shown) connected to conductor |02, in exactly the same way as described for the transmitter, with reference to Fig. 2.

The receiver is brought and locked properly into step with the transmitter by the following process. The frequency of the .pulse train at the output of the divider 58 is divided again by 8 the divider E03. The divided train, whose pulses are separated by 8 channel periods, is passed through a delay network |04 and an open gating circuit G3 to the combining circuit 51 which operates to insert extra pulses, which come from the last-mentioned train, into the train of digit pulses produced by the pulse generator 56. It will be appreciated that one extra pulse occurs `ser eight channel periods, and this extra .pulse causes the pulse divider 58 effectively to shorten the channel period by one digit period. It will be seen that the eiiect of this is to cause the marked digit pulses to step along the channel period until they occurat the beginning of the channel period. Then the gating circuit 63 is shut and the receiver remains in step with the transmitter. For the proper working of this arrangement the delay network 04 should be adjusted to cause the extra pulses to occur about hall" way between two of the digit pulses. Alternatively, the combining circuit 5l could be arranged to suppress one digit pulse in every eight channel periods.

The marked digit pulses are recognized by the fact that they occur regularly at half the channel irequency, namely, at 4,000 .pulses per second. The selecti-on of these digit pulses is performed by a gating circuit 105 to which the incoming digit pulses obtained from conductor 09 are fed, t0- gether with gating pulses of the same duration obtained from unit 58. The gating pulses, which are repeated at the channel frequency of 8,000 pulses per second, mark the commencements of the channel periods, and therefore admit those pulses of the received train which occur at the beginning of these periods and pass them to the band-pass filter @il to pass only a frequency oi' l1.000 cycles per second corresponding to half of the signal channel repetition frequency. The filter can then only produce an output when the marked digit ypulses are received, and this output is rectied by the rectier 65 and passed through an amplitude limiter 5E, and is used as a control voltage to shut the gating circuit 63. This cuts off the extra pulses so that the divider 58 then continues to divide regularly by 64, and

receiver remains properly in step.

It will be noted that after the pulse divider 58 has been stepped on by an extra pulse, the con ditions remain constant for eight channel periods, so that time is given for the marked digit pulses to be inspected and recognised by the lter 64.

Synchronisation of the auxiliary signal channels is obtained directly from the sine wave signal modulated on the synchronising channel. This sine wave is passed through a band-pass iilter 68, designed to accept the synchronising frequency of 2,000 cycles which, as already stated, is transmitted over the synchronising channel S and is obtained from the synchronising channel output conductor |06 of the main signal channel distributor 42, Fig. 4. This sine wave is used to control a generator l0? which is arranged t0 generate a gating pulse train of the same frequency consisting of pulses of the required duration slightly less than one channel period. The four auxiliary channel gating pulse trains I, II, III and IV are then obtained from delay network |08, which is similar to the network QS of Fig. 6.

One form of the anticipator circuit 32 shown in Fig. l is shown in Fig. 8. The purpose of this circuit is to anticipate the variations in the general level of the signal, and its action is based on the principle that if, for example, the level is increasing, the average degree of compression over the relatively long period between two adjustments should be slightly greater than the degree which would be indicated at the beginning of the period.

Accordingly, the anticipator circuit includes an arrangement for differentiating the signal wave and adding a suitable proportion of the differentiated wave voltage to the original wave voltage, so that if, for example, the' level is increasing the voltage which controls the compression will be slightly increased by the added dirrerential component. Likewise, if the level is decreasing the control voltage will be slightly reduced because the diierential component will be negative.

A similar result can be obtained by omitting the anticipator circuit 32 in Fig. 1, and instead of increasing the delay introduced by the delay network 2'! by, say, two channel periods.

In Fig. 8, the incoming signal is applied to resistances 'I6 and 'il which form an input padding circuit. Resistance in parallel with condenser l forms a dierentiating circuit, and the desired output signal, which is the sum of the original and differential signals is obtained from the output pad formed by resistances 'I9 and 8l.

It will be understood that although a specific arrangement has been described to illustrate the invention, Various other arrangements are possible. It is not essential that the number of channels of the system should be sixteen, or that three auxiliary channels each serving four communication channels should be used, or even that they should all serve the same number of channels. However, if any auxiliary channel serves n communication channels then any corresponding auxiliary distributor (such as. 2| or 37 in Fig. 1) should operate at a cyclic frequency which is 1/11. times the operation frequency of the principal distributor such as 26. It is clear also that any suitable frequency could be selected for the channel repetition frequency, for which 8,000 code groups per second was selected as an example.

Further, the invention does not depend on the use of any particular coding system, or a code with any particular number of digits, and any suitable coders and decoders could be used. It has been assumed in Fig. 1 that a common coder is used for all channels, but this is not essential, and if desired, a separate coder could be used for each channel, or for groups of channels, accord ing to known practice.

It is therefore not intended that the examples 17' which have been given shall limit the invention, the scope of which is to be determined from the appended claims.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.'

The embodiments of the invention in which an exclusive property or privilege is claimed are as follows:

1. A multichannel electric pulse code modulation system of communication comprising a transmitter connected over a communication medium to a receiver, in which there is provided at the transmitter an amplitude compressor for each channel' through which a corresponding communication signal wave is applied to a pulse code modulator for generating a train of channel code groups of pulses, means for periodically adjusting the degree of compression, and means for generating an auxiliary train of code groups of pulses conveying a control signal representing the variations in the degree of compression applied to the communication signal Wave, the transmitter also including means for interleaving all the train of code groups of pulses and for transmitting them over the medium; and in which there is provided at the receiver means for receiving all the trains from the medium and for separating them from one another, and also for each channel there is provided means for demodulating the corresponding channel train of code groups, means for passing the demodulated channel signal through an amplitude expander, means for demodulating the corresponding auxiliary train of code groups, and means for applying the demodulated control signal to adjust the ampltude expander in such manner as to neutralise the compression produced in the transmitter.

2. A system according to claim 1 in which the amplitude compressor comprises a circuit having variable attenuation.

3. A system according to claim 1 in which the amplitude compressor comprises a variable gain amplifier.

4. A system according to claim 3 comprising means at the transmitter for rectifying the signal wave before compression, means for generating pulse code groups representing the voltagevariations of the rectied wave, means for transmitting the code groups to the receiver, means for decoding the code groups to recover the said voltage'and means for applying the recovered voltage to control the gain of the amplier.

5. A system according to claim 3 comprising at the transmitter means for rectifying the signal wave after compression, means for generating pulse code groups representing the voltage variation of the rectified wave, means for transmitting the code groups to the receiver, means for decoding the code groups at thetransmitter to recover the said voltage, means for integrating the recovered voltage, and means for applying the integrated recovered voltage to control thegain of the ampliiier; and atthe receiver means for integrating the demodulated control signal before application to adjust theramplitude expander.

6. A system according to claim 1 in which the amplitude compressor comprises a subtractive circuit adapted to subtract from the signal wave amplitude an amplitude representing the mean level variation of the signal wave.

i8 7. A system according V,to -claimil comprising means at the transmitter ior generating a single auxiliary tram 0I C0016 gIOupS 0T. plllSeS l/OV COIlVey control signals I'or a plurality 'oi' the communication channels,rsuccessive code groups oi' the auxiliary train being arranged to relate respectively to all the said plurality o1' chaniieis in turn; and at the receiver, means ior segregating the code groups relating to the respective cnannels and means i'or separately olemoduiating the segregated code groups and Ior applying eacri or' the demodulateo. control signals to auJust the corresponding amplitude expander.

8. A `system according to claim 7 comprising at the transmitter a principal distributor ior interleaving vthe trains or pulsecode groupscorresponding to the communication channels. to at least two auxiliary channels, and also to a synchronising channel, and comprising at least two auxiliary distributors corresponding respectively to the auxiliary channels, and operating at a cyclic frequency equal to l/n times the cyclic `frequency of the principal distributor, where n is the number oi communication channels served by eacli of the auxiliary channels, each auxiliary distributor being adapted to collect the control signals for the corresponding communication channels and to apply them to produce pulse code groups corresponding in 'turn to the respective communication channels; and further comprising at the receiver a second principal distributor for segregating the trains of pulse code groups corresponding to the channels of the system, and at least two further auxiliary distributors each or" which is adapted to segregate the control signals of the corresponding communication channels; the system further comprising means for transmitting signals over the synchronising channel for synchronising the principal and auxiliary distributors at the receiver respectively with the corresponding distributors at the transmitter.

9. A system according to claim 8 comprising means at the transmitter for generating a train of digit pulses for modulation to form the trains of pulse code groups, means for identifying the first digit element or" each code group corresponding to the synchronising channel, means for modulating the synchronising channel with a periodic signal having the periodicity of the auxiliary distributors, means at the receiver for recognising the identified ndigit elements for bringing the principal distributor at the receiver into step with that at the transmitter, and means for employing the periodic signal to bring the auxiliary distributors at the receiver respectively into step with those at the transmitter.

10. A system according to claim 8 comprising at the transmitter at least two additional auxiliary distributors corresponding respectively with the first mentioned auxiliary distributors and operating at the same cyclic frequency, each of which additional distributors is adapted to segregate the control signals decoded at the transmitter and to apply them respectively to the amplitude compressors of the corresponding communication channels, and a selecting distributor operating at the same cyclic frequency as the principal distributor and adapted to direct the decoded control signals to the appropriate additional auxiliary distributor.

BENT BULOW JACOBSEN.

(References on following page) 25;*5`88825 19 2G'v REFERENCESt CITED The following references are o frrecordv in the FOREIGN PATENTS ieqf this patent: Number Country Date UNITED STATES PATEN'I'Sv 249,263 Switzerland Apr.- 1, 1948 Number: Naim@ Date 5 OTHER- REFERENCES Mertrz' Jan; 2, L t 2213-3933 Bennett sept. 3, 1940 26268- ahora ones Record July 1947 pages 'fggg --Lb'g' Electronics, December 1947, pages 126-131'.

2,437,707 Pierce Mar; 16, 1943 m gqgabora Ory Record December 1948 pages 2,438,908 Goodall Apr. 6, 194:8 B 11 I tem Te h 2,449,467 Goodall sept-.- 14; 194s pags c meal Journal January 1948 2,451,044 Pierce Oct. 12, 1943 n 2;453,461 Schelleng NOV. 9, 1948 l" 2;464,60' Pierce Mar. 15, 1949 

